# Diode

File:Diode-photo.JPG
Types of diodes

A diode can be thought of as the electronic version of a check valve. By restricting the direction of movement of charge carriers, it allows an electric current to flow in one direction, but essentially blocks it in the opposite direction.
Diodes may be made from semiconductor materials such as silicon or germanium or may be fabricated using devices depending on thermionic emission: tubes (US) (or valves in UK).

## Diode technology

The first diodes were vacuum tube devices (also known as thermionic valves), arrangements of electrodes surrounded by a vacuum within a glass envelope, similar in appearance to incandescent light bulbs. The arrangement of a filament and plate as a diode was invented in 1904 by John Ambrose Fleming (scientific adviser to the Marconi Company) based on an observation by Thomas Edison. Like light bulbs, vacuum tube diodes have a filament through which current is passed, heating the filament. When heated, the filament can emit electrons into the vacuum. These electrons are electrostatically drawn to a positively charged outer metal electrode called the anode, or "plate". Few electrons flow from the plate back toward the filament, even if the charge on the plate is made negative, because the plate is not heated and therefore does not eject many electrons by thermionic emission.

Although vacuum tube diodes are still used for a few specialized applications, most modern diodes are based on semiconductor p-n junctions. In a p-n diode, conventional current can flow from the p-type side (the anode) to the n-type side (the cathode), but not in the opposite direction. When the diode is reverse-biased, the charge carriers are pulled away from the center of the device, creating a depletion region.

## Physical explanation of semiconductor diode operation

A semiconductor diode's current-voltage, or I-V, characteristic curve is ascribed to the behavior of the so-called Depletion Layer or Depletion Zone which exists at the p-n junction between the differing semiconductors. When a p-n junction is first created, conduction band (mobile) electrons from the N-doped region diffuse into the P-doped region where there is a large population of holes (places for electrons in which no electron is present) with which the electrons "recombine". When a mobile electron recombines with a hole, the hole vanishes and the electron is no longer mobile. Thus, two charges carriers have vanished. The region around the p-n junction becomes depleted of charge carriers and thus behaves as an insulator. However, the Depletion width cannot grow without limit. For each electron-hole pair that recombines, a positively-charged dopant ion is left behind in the N-doped region, and a negatively charged dopant ion is left behind in the P-doped region. As recombination proceeds and more ions are created, an increasing electric field develops through the depletion zone which acts to slow and then finally stop recombination. At this point, there is a 'built-in' potential across the depletion zone. If an external voltage is placed across the diode with the same polarity as the built-in potential, the depletion zone continues to act as an insulator preventing a significant electric current. However, if the polarity of the external voltage opposes the built-in potential, recombination can once again proceed resulting in substantial electric current through the p-n junction. For silicon diodes, the built-in potential is approximately 0.6 V. Thus, if an external current is passed through the diode, about 0.6 V will be developed across the diode such that the P-doped region is positive with respect to the N-doped region and the diode is said to be 'turned on'.

File:Rectifier vi curve.GIF
I-V characteristics of a P-N junction diode (not to scale).

A diode's I-V, characteristic can be approximated by two regions of operation. Below a certain difference in potential between the two leads, the Depletion Layer has significant width, and the diode can be thought of as an open (non-conductive) circuit. As the potential difference is increased, at some stage the diode will become conductive and allow charges to flow, at which point it can be thought of as a connection with zero (or at least very low) resistance. More precisely, the transfer function is logarithmic, but so sharp that it looks like a corner (see also signal processing).

The Shockley ideal diode equation (named after William Bradford Shockley) can be used to approximate the p-n diode's I-V characteristic.

$\displaystyle I=I_\mathrm{S} \left( {e^{qV_\mathrm{D} \over nkT}-1} \right)\,$ ,

where I is the diode current, IS is a scale factor called the saturation current, q is the charge on an electron (the elementary charge), k is Boltzmann's constant, T is the absolute temperature of the p-n junction and VD is the voltage across the diode. The term kT/q is the thermal voltage, sometimes written VT, and is approximately 26 mV at room temperature. n (sometimes omitted) is the emission coefficient, which varies from about 1 to 2 depending on the fabrication process and semiconductor material.

It is possible to use a shorter notation. Putting

$\displaystyle \frac{k T}{q} = V_\mathrm{T}$

and $\displaystyle n=1$ the relationship of the diode becomes:

$\displaystyle I=I_\mathrm{S} \left( {e^{V_\mathrm{D} \over V_\mathrm{T}}-1} \right)\,$

where $\displaystyle V_\mathrm{T} = 26 mV$ (at room temperature) is a known constant.

In a normal silicon diode at rated currents, the voltage drop across a conducting diode is approximately 0.6 to 0.7 volts. The value is different for other diode types - Schottky diodes can be as low as 0.2 V and light-emitting diodes (LEDs) can be 1.4 V or more depending on the current.

Referring to the I-V characteristics image, in the reverse bias region for a normal P-N rectifier diode, the current through the device is very low (in the µA range) for all reverse voltages upto a point called the peak-inverse-voltage (PIV). Beyond this point a process called reverse breakdown occurs which causes the device to be damaged along with a large increase in current. For special purpose diodes like the avalanche or zener diodes, the concept of PIV is not applicable since they have a deliberate breakdown beyond a known reverse current such that the reverse voltage is "clamped" to a known value (called zener voltage). The devices however have a maximum limit to the current and power in the zener or avalanche region.

## Types of semiconductor diode

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There are several types of semiconductor junction diodes:

Normal (p-n) diodes
which operate as described above. Usually made of doped silicon or, more rarely, germanium. Before the development of modern silicon power rectifier diodes, cuprous oxide and later selenium was used; its low efficiency gave it a much higher forward voltage drop (typically 1.4-1.7V per "cell," with multiple cells stacked to increase the peak inverse voltage rating in high voltage rectifiers), and required a large heat sink (often an extension of the diode's metal substrate), much larger than a silicon diode of the same current ratings would require.
'Gold doped' diodes
The gold causes 'minority carrier suppression.' This lowers the effective capacitance of the diode, allowing it to operate at signal frequencies. A typical example is the 1N914. Germanium and Schottky diodes are also fast like this, as are bipolar transistors 'degenerated' to act as diodes. Power supply diodes are made with the expectation of working at a maximum of 2.5 x 400 Hz (sometimes called 'French power' by Americans), and so are not useful above a kilohertz.
Zener diodes (pronounced /ziːnər/
diodes that can be made to conduct backwards. This effect, called Zener breakdown, occurs at a precisely defined voltage, allowing the diode to be used as a precision voltage reference. Some devices labeled as high-voltage Zener diodes are actually avalanche diodes (see below). Two (equivalent) Zeners in series and in reverse order, in the same package, constitute a transient absorber (or Transorb, a registered trademark). They are named for Dr. Clarence Melvin Zener of Southern Illinois University, inventor of the device.
Avalanche diodes
diodes that conduct in the reverse direction when the reverse bias voltage exceeds the breakdown voltage. These are electrically very similar to Zener diodes, and are often mistakenly called Zener diodes, but break down by a different mechanism, the Avalanche Effect. This occurs when the reverse electric field across the p-n junction causes a wave of ionization, reminiscent of an avalanche, leading to a large current. Avalanche diodes are designed to break down at a well-defined reverse voltage without being destroyed. The difference between the avalanche diode (which has a reverse breakdown above about 6.2 V) and the Zener is that the channel length of the former exceeds the 'mean free path' of the electrons, so there are collisions between them on the way out. The only practical difference is that the two types have temperature coefficients of opposite polarities. Practical voltage reference circuits feature Zener and switching diodes connected in series and opposite directions to balance the temperature coefficient to near zero.
Transient voltage suppression (TVS) diodes
These are avalanche diodes designed specifically to protect other semiconductor devices from electrostatic discharges. Their p-n junctions have a much larger cross-sectional area than those of a normal diode, allowing them to conduct large currents to ground without sustaining damage.
Light-emitting diodes (LEDs)
as the electrons cross the junction they emit photons. In most diodes, these are reabsorbed, and are at frequencies that can not be seen (usually infrared). However, with the right materials and geometry, the light becomes visible. The forward potential of these diodes define their color. Thus different materials (extrinsic semiconductors) must be used. 1.2 V corresponds to red, 2.4 to violet. Now, even soft UV diodes are available. The first LED's were red and yellow, and higher-frequency diodes have been developed over time. Polishing the device with parallel faces, so as to form a resonant cavity, yields a 'laser diode.' All LEDs are monochromatic; 'white' LED's are actually combinations of three LED's of a different color, or a blue LED with a yellow scintillator coating. The lower the frequency of emission, the greater the efficiency, so to normalize output when using LED's of different colors it is necessary to increase current in the higher frequency models. This effect is complicated somewhat by the fact that the human eye is most sensitive in the blue-green.
Photodiodes
these have wide, transparent junctions. Photons can push electrons over the junction, causing a current to flow. Photo diodes can be used as solar cells, and in photometry. If a photon doesn't have enough energy, it will not overcome the band gap, and will pass through the junction. LED's can be used as low-efficiency photodiodes in signal applications. Sometimes a LED is paired with a photodiode or phototransistor in the same package. This device is called an "opto isolator." Unlike a transformer, this scheme allows for DC coupling. These are used to protect hospital patients from shock. Patients with IV's in their bodies are particularly susceptible, sometimes succumbing to 'carpet shock.' They are also used to isolate low-current control or signal circuitry from "dirty" power supply circuits or higher-current motor and machine circuits.
Schottky diodes
have a lower forward voltage drop than a normal PN junction, because they are constructed from a metal to semiconductor contact. Their forward voltage drop at forward currents of about 1 mA is in the range 0.15V to 0.45 V, which makes them useful in voltage clamping applications and prevention of transistor saturation. They can also be used as low loss rectifiers although their reverse leakage current is generally much higher than non Schottky rectifiers. Schottky diodes are majority carrier devices and so do not suffer from minority carrier storage problems that slow down most normal diodes. They also tend to have much lower junction capacitance than PN diodes and this contributes towards their high switching speed and their suitability in high speed circuits and RF devices such as mixers and detectors.
Snap-off or 'step recovery' diodes
The term 'step recovery' relates to the form of the reverse recovery characteristic of these devices. After a forward current has been passing in an SRD and the current is interruped or reversed, the reverse conduction will cease very abruptly (as in a step waveform). SRDs can therefore provide very fast voltage transitions by the very sudden disappearance of the charge carriers.
Esaki or tunnel diodes
these have a region of operation showing negative resistance caused by quantum tunneling, thus allowing amplification of signals and very simple bistable circuits. These diodes are also the type most resistant to nuclear radiation.
Gunn diodes
these are similar to tunnel diodes in that they are made of materials such as GaAs or InP that exhibit a region of negative differential resistance. With appropriate biasing, dipole domains form and travel across the diode, allowing high frequency microwave oscillators to be built.

There are other types of diodes, which all share the basic function of allowing electrical current to flow in only one direction, but with different methods of construction.

Point Contact Diode
This works the same as the junction semiconductor diodes described above, but its construction is simpler. A block of n-type semiconductor is built, and a conducting sharp-point contact made with some group-3 metal is placed in contact with the semiconductor. Some metal migrates into the semiconductor to make a small region of p-type semiconductor near the contact. The long-popular 1N34 germanium version is still used in radio receivers as a detector and occasionally in specialized analog electronics.
Varicap or varactor diodes
These are used as voltage-controlled capacitors. These were important in PLL (phase-locked loop) and FLL (frequency-locked loop) circuits, allowing tuning circuits, such as those in television receivers, to lock quickly, replacing older designs that took a long time to warm up and lock. A PLL is faster than a FLL, but prone to integer harmonic locking (if one attempts to lock to a broadband signal). They also enabled tunable oscillators in early discrete tuning of radios, where a cheap and stable, but fixed-frequency, crystal oscillator provided the reference frequency for a voltage-controlled oscillator.
Current-limiting field-effect diodes
These are actually a JFET with the gate shorted to the source, and function like a two-terminal current-limiting analog to the Zener diode; they allow a current through them to rise to a certain value, and then level off at a specific value. Also called CLDs, constant-current diodes, or current-regulating diodes. [1], [2]

Other uses for semiconductor diodes include sensing temperature, and computing analog logarithms.

## Thermionic or gaseous state devices

Tube or Valve Diode
This is the simplest kind of vacuum tube device (referred to as a valve in the UK). Electrons will move from a heated metal surface (cathode) treated with a mixture of barium and strontium oxides into a vacuum (thermionic emission). After leaving the cathode, they can be attracted to positively charged cool surface (anode). However, electrons are not easily released from a cold untreated surface when the voltage polarity is reversed and hence any flow is a very small current. For much of the 20th century they were used in analog signal applications, and as rectifiers in power supplies. Tube diodes were nearly obsolete by 2001, except as rectifiers in tube guitar and hi-fi amplifiers and in a few specialized high-voltage applications.

## Applications

The first use for the diode was the demodulation of amplitude modulated (AM) radio broadcasts. The history of this discovery is treated in depth in the radio article. In summary, an AM signal consists of alternating positive and negative peaks of voltage, whose amplitude or 'envelope' is proportional to the original audio signal, but whose average value is zero. The diode rectifies the AM signal (i.e. it eliminates peaks of one polarity), leaving a signal whose average amplitude is the desired audio signal. The average value is extracted using a simple filter and fed into an audio transducer (originally a crystal earpiece, now more likely to be a loudspeaker), which generates sound.

### Power conversion

A half wave rectifier can be constructed from a single diode where it is used to convert alternating current electricity into direct current, by removing either the negative or positive portion of the AC input waveform.

A special arrangement of four diodes that will transform an alternating current into a direct current, using both positive and negative excursions of a single phase alternating current, is known as a diode bridge, single-phase bridge rectifier, or simply a full wave rectifier.

With a split (center-tapped) alternating current supply it is possible to obtain full wave rectification with only two diodes. Often diodes come in pairs, as double diodes in the same housing.

When it is desired to rectify three phase power, one could rectify each of the three phases with the arrangement of four diodes used in single phase, which would require a total of 12 diodes. However, due to redundancy, only six diodes are needed to make a three phase full wave rectifier. Most devices that generate alternating current (such devices are called alternators) generate three phase alternating current.

File:Getting behind the tridge rectifier.jpg
Disassembled automobile alternator, showing the six diodes that comprise a full-wave three phase bridge rectifier.

For example, an automobile alternator has six diodes inside it to function as a full wave rectifier for battery charge applications.

### Over-voltage protection

Diodes are frequently used to conduct damaging high voltages away from sensitive electronic devices. They are usually reverse-biased (non-conducting) under normal circumstances, and become forward-biased (conducting) when the voltage rises above its normal value. For example, diodes are used in stepper motor and relay circuits to de-energize coils rapidly without the damaging voltage spikes that would otherwise occur. Many integrated circuits also incorporate diodes on the connection pins to prevent external voltages from damaging their sensitive transistors. Specialized diodes are used to protect from over-voltages at higher power (see Diode types above).

### Logic gates

Diodes can be combined with other components to construct AND and OR logic gates.